1. Field of the Invention
The invention relates to camera apparatus for determining if the camera is adjusted to achieve a suitable exposure for prevailing scene light conditions. More particularly, the invention provides exposure control apparatus which takes into account the exposure latitude of the film being utilized in the camera in determining whether or not the camera is adjusted to achieve a suitable exposure.
2. Description Relative to the Prior Art
Cameras are known in the art having exposure control circuits that deflect a movable member such as a needle in relation to the amount of light present and the light sensitivity, frequently called speed, of the film being used. The light is sensed by a photoconductive device, and the film speed value is typically provided by manual adjustment of a multi-position switch. The camera further includes a reference mark mechanically coupled to the aperture and/or shutter control mechanism, the mark being located in proximity to the needle. The camera operator views the deflecting needle and the reference mark and manually adjusts the aperture and/or shutter mechanism until the needle and reference mark coincide. With such an adjustment, an exposure may be obtained that produces a quality photograph.
There are other cameras that provide for proper camera adjustment by use of a balancing circuit, such as a Wheatstone bridge, one arm of the bridge including a photocell, and a second or reference arm including electrical elements having values which are functionally related to the shutter speed and/or aperture size at which the camera is adjusted. When a balanced electrical condition is achieved between the arms, the camera is considered to be properly adjusted.
A disadvantage of these kinds of cameras is that only one particular exposure value is used to determine if they are properly adjusted.
U.S. Pat. Nos. 3,678,827 and 3,852,777, both of which are assigned to the assignee of the present invention, disclose camera apparatus which control low-light signal mechanisms in accordance with the underexposure latitude of films in the cameras. The '777 patent discloses an automatic exposure control apparatus for a camera that takes into account underexposure latitude to determine if a suitable exposure may be made in dim light conditions without increasing the exposure time beyond a predetermined unacceptable limit. When a light level is detected that requires an excessively long exposure time to achieve an "optimum" exposure level, but is within the limits of the underexposure latitude of the film, the exposure control causes an exposure to be effected that is less than "optimum" without increasing exposure time beyond the predetermined limit. In doing so, a low-light indicator is inhibited when underexposure latitude is taken into account.
The apparatus disclosed in both the '827 and '777 patents permits the low-light capability of the cameras to be expanded. However, such apparatus does not utilize overexposure latitude of photographic films to expand a camera's apparent usefulness for making acceptable exposures of very bright scenes.
With many negative films, acceptable photographs may be achieved even though the film is overexposed or underexposed from what might be considered an "optimum" exposure. Some films of this type, for example, may be overexposed up to the equivalent of three exposure values, or underexposed the equivalent of two exposure values, yet an acceptable photograph will be produced. Using these films, one camera setting will suffice for many normally encountered scene conditions. For example, a camera using a particular negative film may be set for an exposure appropriate for overcast or open shade conditions and still acceptably accommodate exposure conditions ranging from bright indoor daylight scenes to summer sunlight scenes. In effect, these exists, instead of a discrete "optimum" exposure, a "window", constituting a range of exposures, appropriate for the film being utilized, that produces satisfactory photographs. As long as the camera is adjusted to produce an exposure within this "window", the camera operator may make photographs without concern for certain departures from the "optimum" exposure. Therefore, it is apparent that the prior art camera apparatus discussed above for indicating the appropriateness of camera settings are unnecessarily restricted.